Aromatic carbonate polymers are well known, commercially available materials having a variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl)propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. See the Encyclopedia of Polymer Science and Technology, Vol. 10, pp. 710-764, Interscience, New York, 1969, which is incorporated herein by reference. Generally speaking, aromatic polycarbonate resins offer a high resistance to attack by mineral acids, and they are phyiologically harmless as well as stain resistant. In addition, articles molded from such polymers have a high tensile strength and a high impact strength, except in thick sections, a high heat resistance and a dimensional stability far surpassing that of most other thermoplastic material. However, in certain applications, the use of aromatic polycarbonate resins is limited because (i) they have a high viscosity in the melt, making molding of complex, large, and especially foamed parts difficult; (ii) they exhibit brittleness under sharp impact conditions in thick sections and regardless of thickness when small amounts of reinforcements, e.g., glass or pigments, e.g., titanium dioxide, are added for conventional purposes; and (iii) they exhibit severe environmental stress crazing and cracking. The term "environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents, e.g., acetone, heptane and carbon tetrachloride when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. Such contacts may occur, for example, when the solvents are used to clean or degrease stressed parts fabricated from polycarbonates, or when such parts are used in automobiles, especially under the hood.
The relatively high melt viscosities and softening points of aromatic polycarbonates make them difficult to melt process and several approaches have been suggested for improving melt flow, but they have disadvantages. For example, plasticizers can be added but other important properties are lost, the parts becoming brittle and losing a substantial amount of their ability to resist distortion by heat. On the other hand, as is suggested in Goldblum, U.S. Pat. No. 3,431,224, small amounts of polyethylene can be added, and, while this markedly enhances resistance to environmental stress cracking, low levels of polyethylene are not too effective to enhance melt flow and an increase into effective ranges tends to result in molded articles which delaminate.
In co-pending, commonly-assigned application Ser. No. 833,364, pending in Group 140 it is reported that the addition of a minor amount of a hydrogenated block copolymer to aromatic polycarbonates causes the melt viscosity to go way down, but the heat distortion temperature is substantially unaffected. It is further reported that adding hydrogenated block copolymers to polycarbonates leads to improvement in impact resistance in thick-walled molded articles. A third major advantage reported after adding hydrogenated block copolymers to polycarbonates is to improve their environment resistance. Thus, the molded parts can be subjected to more strain before cracking starts, without appreciably affecting any other of their desirable properties.
Compositions comprising linear block copolymers of the A-B-A type and aromatic polycarbonates are also described in Gergen et al., U.S. Pat. No. 4,088,711. In Gergen et al., U.S. Pat. No. 4,090,996, there are described such compositions which also include a saturated thermoplastic polyester which is further characterized as having a generally crystalline structure and a melting point over about 120.degree. C.
The present invention is a departure from and an improvement over the above-mentioned patents and application, in which the components are intimately admixed in carefully selected ratios and there is used an amorphous, instead of crystalline, saturated thermoplastic polyester component.
The compositions contemplated by the present invention are restricted to those within the following network; the selectively hydrogenated block copolymer (A-B-A as well as radial teleblock), 0.1-6 pbw, preferably 1-4 pbw; aromatic polycarbonate 65-97.5 pbw, preferably 76-88 pbw; and amorphous saturated thermoplastic polyester resin, 1-30 pbw, preferably 10-20 pbw.
The data in the above-mentioned U.S. Pat. No. 4,090,996 indicates the need to use relatively high loadings of A-B-A block copolymers together with a high ratio of crystalline polyester to polycarbonate (greater than 1:1 polyester to polycarbonate). At lower loadings of A-B-A block copolymers, it has been found that high ratio crystalline polyester-polycarbonate blends are almost impossible to extrude without unacceptable die swell, etc. The problem with using higher loadings of A-B-A block copolymers, instead of 6% or less herein, however, is loss of mechanical properties such as creep, tensile modulus and deflection temperature under load. To solve the processability and property loss problems, applicants herein control the polyester content to lower levels of up to 30%, but preferably 20%, i.e., the ratio of polyester to polycarbonate is less than 1:1 and they use an amorphous polyester instead of the crystalline polyester of the prior art composition. This permits the block copolymers to be added, not only as processing aids, but also to improve stress crack resistance, cold temperature impact strength, and for achieving a more predictable ductile-brittle (DB) impact transition. Such objects and advantages are in no way suggested by U.S. Pat. No. 4,090,996. Moreover, the use of the amorphous polyester provides the improved properties of the compositions.
The new compositions may also be reinforced, e.g., with fibrous glass, and rendered flame retardant either by using a halogenated aromatic polycarbonate as all or part of component (b), and/or by using flame retardant additives, or they may be pigmented, and/or foamed by known procedures to extend their field of use in melt processed products.
In comparison with the compositions of prior art, they will in general, also have high stiffness and strength, excellent surface appearance, and excellent resistance to discoloration by heat.